In order to preserve foodstuffs, food, organisms, and other materials with freshness maintained for a long time, frozen storage has been conventionally applied. However, known frozen storage cannot perfectly prevent changes in color tone of frozen materials, deterioration in taste, and occurrence of dripping. More specifically, it cannot prevent deterioration of the quality or freshness due to, for example, dripping resulting from thawing. Materials to be frozen, such as foodstuffs, food products, and organisms, contain a large amount of water. The water contained in these materials is constituted of bound water tied to protein or other molecules in the materials and free water freely transferring in the materials without being tied to the molecules. In freezing, the free water is frozen, so that ice crystals grow. If the ice crystals grow coarse, the cells of the materials are destructed. In foodstuffs and food products, and so forth, once the cells of the material are destructed, dripping, which makes it difficult to restore the organisms to the original state, occurs while the material is thawed.
Coarseness of the ice crystals is caused at freezing by a slowly passing through the range of ice crystallization temperature. Accordingly, in order to prevent such coarseness of the ice crystals, the material to be frozen may be immersed in a liquid cooling medium or a liquid cooling medium may be sprayed onto the material so that the material temperature may quickly passes through the range of ice crystallization temperatures to cool the material rapidly. Although the process of immersing the material to be frozen into a liquid cooling medium or spraying a liquid cooling medium onto the material can cool the surface layer of the material quickly, a frozen layer is produced in the surface layer. The cooling rate of the inner side of the frozen material is determined by heat transfer from the surface, so the frozen layer in the surface layer interferes the heat transfer to the inner side, thus the cooling of the inner side of the frozen material is delayed. Consequently, the coarseness of the ice crystals disadvantageously occurs in the inside of the frozen material; hence, the coarseness of the ice crystals is not prevented.
To solve the problem, a method for super-quickly cooling has been disclosed in, for example, WO 01/24647 A1. This method has the step of quickly freezing an object by lowering the ambient temperature of the object to a temperature in the range of −30 to −100° C. while a unidirectional magnetic field is applied to the object or further the step of cooling the object with cold air flow at 1 to 5 m/s and applying a sound wave in the audio frequency band to the cold air flow, or further includes the step of applying an electric field to the object.
WO 01/24647 A1 also proposes a super-quickly freezing apparatus. This apparatus comprises a freezer capable of lowering its internal temperature around the object to be freezed in the range of −30 to −100° C.; and magnetic field-generating means for applying a magnetic field fluctuating in one direction to the object, comprising static magnetic field-generating means and dynamic magnetic field-generating means.
According to the technique described in WO 01/24647 A1, the cells of the frozen material are prevented from being destructed and the food after thawing has a taste similar to its raw state. Thus, the quality of food preserved in frozen storage is fairly improved. In some types of food, however, the destruction of the cells cannot be completely prevented even by this technique, and quality degradation occurs undesirably in the food frozen. In addition, the inventor of the present invention found in the technique of WO 01/24647 A1 that the variable magnetic field is so nonuniform that the effect of the variable magnetic field is not evenly exerted on the frozen material, and that a part of the frozen food is degraded in quality.
In view of these problems, an object of the present invention is to provide a high-functional freezing apparatus and a high-functional freezing method that allow any type of food product, foodstuff, and organism without destruction of their cells to be preserved in frozen storage.
Another object of the present invention is to provide a high-functional freezing apparatus containing means for simultaneously applying an electric field of variable frequency and a uniform magnetic field to food and organism cells, and a high-functional freezing method.